Planar light source and method for forming a light scatterer for a planar light source

ABSTRACT

A plane display lamp using one or two dot-like light sources. This plane display lamp comprises a plate-like light guiding member; a bottom plate covering a bottom surface of said light guiding member; a casing frame covering side faces of said light guiding member; a diffusing sheet covering a top surface of said light guiding member; and dot-like light sources provided on one side face or on two side faces opposite to each other of said light guiding member and emitting light to said light guiding member, and a light scatterer pattern is provided on a bottom surface of the light guiding member. When a side face thereof is inclined, a light scatterer pattern may be provided also on the inclined side face or just below the inclined side face.

TECHNICAL FIELD

[0001] The present invention relates to a plane display lamp. Morespecifically this invention relates to a plane display lamp based on alight guiding system in which a bottom surface of a light guiding memberhas a light scattering function for homogeneously emitting light to theoutside, and further to a method of a light scatterer pattern realizinga light scattering function.

BACKGROUND ART

[0002] A plane display lamp (with the display area of several tens cm²or less) are used in various types of devices and equipment used indoorsto display a state of or an instruction for an operation of the devicesor equipment. One example of the plane display lamp is a pilot lamp inwhich a light source such as a compact incandescent lamp or alight-emitting diode (LED) is attached to a casing having a coloredplastic plate surface. In these types of lamps, when a letter or a signis to be displayed on a display surface, as the luminance just above thelight source is higher than that in the peripheral area of the displaysurface, non-uniformity in luminance on the display surface is large,and the letter or sign displayed on the surface can not well be visuallyrecognized. Further these types of display lamps allows onlydiscrimination between two states basically by turning ON or OFF thelamp. Also there is a plane display lamp in which a light source capableof emitting light changing its color is provided under asemi-transparent plastic plate, but non-uniformity in luminance variesaccording to a color of emitted light more remarkably as compared to thecase of single color plane display lamp, so that the adaptability to bevisually recognized is further lower.

[0003] As a means for reducing non-uniformity in luminance, there hasbeen known the method used in a planar illumination device applied as aback light for a liquid crystal display unit. In this method, aplate-like light guiding member is used as a planar light source, and alinear light source such as a fluorescent light source or a plurality ofdot-like light sources such as LEDs are provided around the plate-likelight guiding member. Generally a reflection layer with distributedreflectivity is provided on a bottom surface of the light guiding memberso that the light sources emit light with substantially homogeneousintensity from a surface of the plate-like light guiding member.

[0004] In the pilot lamp with the conventional type of light source suchas an incandescent lamp or an LED attached therein as described above,non-uniformity in luminance is large on the light-emitting surface, sothat the visibility of displayed contents is low and also the displayquality is disadvantageously low.

[0005] Further with the plane display lamp having a light source capableof emitting light changing its color to provide light with a desiredcolor, it is impossible to obtain the excellent display quality.

DISCLOSURE OF THE INVENTION

[0006] It is an object of the invention to provide a plane display lampwith reduced non-uniformity in luminance by using a plate-like lightguiding member.

[0007] It is another object of the invention to provide a plane displaylamp with reduced power consumption and having a light source which caneasily be driven.

[0008] It is still another object of the invention to provide a planedisplay lamp having two dot-like light sources.

[0009] It is another object of the present invention to provide a planedisplay lamp having only one dot-like light source, yet capable ofreducing non-uniformity in luminance on a light-emitting surface.

[0010] It is another object to provide a method of forming a lightscatterer pattern printed on a bottom surface of a plate-like lightguiding member.

[0011] In a first aspect of the present invention, one dot-like lightsource is provided at a center of each of the opposite side faces of arectangular plane light guiding member respectively. To reducenon-uniformity in luminance of light coming out from a surface of thislight guiding member, a light scatterer pattern with reflected lightintensity two-dimensionally distributed thereon is formed on a bottomsurface of the light guiding member.

[0012] As the dot-like light source, one LED chip loaded on the samepackage or a plurality of LED chips, for instance, for three RGBelementary colors loaded adjacent to each other on the same package maybe used. In the latter case, as a plurality of LEDs are providedadjacent to each other, when light for each color is emitted,non-uniformity in luminance does not substantially change, so that aplurality of states can be displayed by using lights for differentcolors.

[0013] What is important in the present invention is a light scattererpattern formed on a rear side of a light-emitting surface of a lightguiding member so that light emitted from a dot-like light source istaken by a plate-like light guiding member to give uniform brightness,namely uniform luminance on the light-emitting surface of the lightguiding member. By giving a certain level of regularity to a patternform, it has become possible to provide homogeneous luminancedistribution on an entire light-emitting surface of a light guidingmember and also to provide planar displays having apparently uniformbrightness.

[0014] To realize the optimal pattern as described above, how to form adot pattern of a light scatterer on a bottom surface of the lightguiding member is important.

[0015] A light scatterer dot pattern formation method according to asecond aspect of the present invention comprises the steps of:

[0016] a) forming a light scatterer homogeneously all over a bottomsurface of a light guiding member for assessment having the same sizeand made from the same materials as said light guiding member;

[0017] b) assembling said plane display lamp;

[0018] c) measuring luminance at each coordinate point on alight-emitting surface of said plane display lamp;

[0019] d) converting the measured luminance at each coordinate pointthrough inverse-proportional operation to standardized value using thelowest value in a distribution the luminance as a reference;

[0020] e) calculating an area of a light scatterer dot at eachcoordinate point from the standardized value obtained from theconversion in the previous step; and

[0021] f) forming a pattern of light scatterer dots each area thereof iscalculated in the previous step on a bottom surface of a light guidingmember different from that for assessment.

[0022] A third aspect of the present invention provides a plane displaylamp which can reduce non-uniformity in luminance on the light-emittingsurface even when only one dot-like light source is used. In the planedisplay lamp using one dot-like light source according to the presentinvention, three of the four side faces of the rectangular planar lightguiding member are inclined side faces. On a central one of these threeincluded side faces, or on a bottom surface of the light guiding memberjust below the side face, one dot-like light source is provided. Furtherprovided on a bottom surface of the light guiding member just below thetop surface of the light guiding member is a light scatterer dot patternformed according to the method according to the second aspect of thepresent invention as described above, and further a light scattererpattern is provided on a bottom surface of the light guiding member justbelow an inclined side face adjacent to an area where light from thedot-like light source does not fully reach and the luminance isinsufficient.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]FIG. 1 is a exploded perspective view showing one example of aplane display lamp according to the present invention;

[0024]FIG. 2 is a view showing a light-emitting surface with a X-Ycoordinate system divided to four fields;

[0025]FIG. 3A and FIG. 3B are views each showing a table of measuredluminance distribution on a plane display lamp in a case where highreflection ink is printed all over a bottom surface of a light guidingmember respectively;

[0026]FIG. 4 is a view showing the luminance distribution in FIG. 3 withthe contour lines;

[0027]FIG. 5A and FIG. 5B are views each showing a table of standardizedvalue distribution respectively;

[0028]FIG. 6A and FIG. 6B are views each showing a table of calculatedcircular dot radiuses;

[0029]FIG. 7 is a view showing a printed circular dot pattern;

[0030]FIG. 8 is a view showing a table of measured luminancedistribution on a plane display lamp in a case where a circular dotpattern is printed on a bottom surface of the light guiding member;

[0031]FIG. 9 is a exploded perspective view showing another example ofthe plane display lamp according to the present invention;

[0032]FIG. 10 is a view showing with contour lines the luminancedistribution when there is provided one dot-like light source on abottom surface of the light guiding member;

[0033]FIG. 11 is a perspective view showing a light guiding memberaccording to the present invention;

[0034]FIG. 12 is a view showing a top surface and a side surface of thelight guiding member shown in FIG. 11;

[0035]FIG. 13 is a view showing an example in which an LED is providedon a bottom surface of a light guiding member;

[0036]FIG. 14 is a view showing another example in which an LED isprovided on a bottom surface of the light guiding member; and

[0037]FIG. 15 is a view showing an example in which a dot-like lightsource is provided inside the light guiding member.

BEST MODE FOR CARRYING OUT THE INVENTION

[0038] First Embodiment

[0039] A exploded perspective view of a plane display lamp according toa first embodiment of the present invention is shown in FIG. 1.

[0040] This plane display lamp 10 has a light guiding member 12comprising a rectangular transparent acrylic resin plate with the sizeof 50 mm×25 mm×5 mm. In this embodiment, each side face of the lightguiding member is vertical to the top surface as well as to the bottomsurface thereof.

[0041] In this light guiding member 12, dot-like light sources 14 a, 14b are provided at central positions of side faces in the oppositeshorter edge sides respectively. An LED with the output of 1.5 mW for aforward direction current of 20 mA and with the main wavelength of greencolor light of 530 nm from Nichia Kagaku Kogyo (K.K) is used as thedot-like light source. The size of one LED (semiconductor chip) is 0.3mm×0.3 mm, and the size of an aperture (light-emitting section) as alight outlet port of the LED package with one chip loaded therein isabout 1 mm×2 mm.

[0042] This transparent acrylic light guiding member 12 has a whitebottom plate 16, four side faces of which are covered with a whitecasing frame 18. A diffusing sheet 20 is adhered on a top surface of,namely in the light-emitting surface side of the white casing frame 18.White ABS resin is used for the white casing 18 as well as for the whitebottom plate 16.

[0043] Further a light scatterer having a predetermined pattern wasformed with the highly reflective white ink from Teikoku Ink Seizou(K.K) on a bottom surface 13 of the light guiding member 12. The patternof the light scatterer is generally a circular dot.

[0044] Although a color of the casing 18 and bottom plate 16 istypically white, other colors such as silver may be used. In the case ofsingle color light source, any appropriate color may be selectedaccording to the wavelength of the light.

[0045] A specific sequence of designing a printed pattern is describedin detail below. At first the light guiding member 12 with the highlyreflective white ink printed on the entire bottom surface thereof isprepared, this light guiding member 12 is placed in the white casing 18and covered with the white bottom plate 16 with a diffusing sheetadhered on a light-emitting surface of the white casing, and then aplane display lamp for designing a light scatterer pattern is assembled.Then the luminance distributed on the light-emitting surface when the 20mA current is flown through each LED is measured.

[0046] As the LEDs 14 a, 14 b are provided at central positions of theside faces 15 a, 15 b in the side of opposite shorter edges of the lightguiding member 12 respectively as described above, when thelight-emitting surface is divided with two lines perpendicular to eachother into four equal regions, the luminance distribution on each regionis identical, so that it is necessary only to measure the luminancedistribution on one of the four regions. FIG. 2 shows a state where thelight-emitting surface is shown with an X-Y coordinate system. Thecoordinate plane is plotted with a corner of the light-emitting surfaceas the origin and the X-axis was divided to 25 equal zones and also theY-axis to 50 equal zones. This figure shows the state where thelight-emitting surface was divided with the two lines 22, 24 crossingeach other at right angles at a center of the light-emitting surface tofour regions. The luminance distribution is measured on the quadrantregion 26 shadowed with inclined lines.

[0047] When the luminance distribution on the region 26 is measured, theluminance distribution on the entire light-emitting surface can beobtained by applying the measured luminance distribution to the otherregions symmetrically in the horizontal direction as well as in thevertical direction against the two lines 22, 24 perpendicular to eachother respectively.

[0048] The measured luminance distribution for the region 26 was shownin FIG. 3A and in FIG. 3B. FIG. 3A and FIG. 3B show tables each showingdistribution of luminance (cd/m²) at each point (x, y) from the originto the coordinate point 12 in the X-axial direction and from the originto the coordinate point 25 in the Y-axial direction. The tables showthat the luminance is lowest at the coordinate point (0, 0) and becomeshigher in the coordinate area close to the LED. Non-uniformity inluminance in this case is 4.4 times when expressed by themaximum/minimum ratio on the plane.

[0049]FIG. 4 shows the luminance distribution on the entirelight-emitting surface based on the tables of the measured luminancedistribution for the region 26 with contour lines 28. From the contourlines indicating the luminance distribution, it can be understood thatthe emitted light from the LED extends in the radial form and theextension angle 2θ is about 60 degrees.

[0050]FIG. 5A and FIG. 5B show tables each showing the standardizedluminance distribution obtained by standardizing the distribution ofmeasured luminance through inverse-proportional conversion using theminimum value in the distribution of luminance shown in FIG. 3A and FIG.3B as a reference. The minimum value in the distribution of luminanceshown in FIG. 3A and FIG. 3B is 98.76 cd/cm² at the coordinate point (0,0), and luminance values at other coordinate points are standardizedthrough inverse-proportional conversion. Namely a standardized value ateach coordinate point is calculated (standardized) through the followingequation:

Standardized value=A×98.76/measured luminance

[0051] wherein A indicates a proportional constant. The obtainedstandardized value is in inverse proportion to the luminance, and as theluminance is higher, the standardized value is lower. The standardizedvalues shown in FIG. 5A and FIG. 5B are those when A is equal to 1.

[0052] The method of allocating a circular dot of a light scatterer toeach coordinate point is described below. FIG. 6A and FIG. 6B are tableseach showing the radiuses of circular dots per unit area (mm²)calculated from the standardized values shown in FIG. 5A and FIG. 5Bthrough the equation of standardized value=A×π×(radius)² assuming that Ais equal to 1 (A=1). Each numerical value in these tables indicates thediameter of a circular dot at each coordinate point, and a plate for aprint pattern is prepared assuming that the diameter of a circular dotis that of a printed circular dot in a square segment with the area of 1mm².

[0053] A pattern of circular dots is printed on a bottom surface of thelight guiding member using the prepared plate for the print pattern.FIG. 7 shows one example of a printed circular dot pattern 30. A solidcircle 32 in this figure shows a circular dot, and the reflectivity inthe circular dot section is high. The light scatterer pattern formedwith a pattern of the circular dots as described above has the featurethat the reflectivity is minimum at the sections “a” and “b” on thesurface.

[0054] A plane display lamp is assembled using the light guiding member12 with the circular dot pattern 30 printed on the bottom surfacethereof. Namely the white bottom plate 16 is adhered to a bottom of thelight guiding member 12, the white casing frame 18 is set therein, andthe diffusing sheet 20 is adhered to the light-emitting surface. In theplane display lamp as described above, the LEDs 14 a and 14 b are turnedON, and the luminance distribution is measured. The measured values areshown in FIG. 8. At the coordinate point (22, 10), the maximum luminancewas 118.9 cd/m², while the minimum luminance at the coordinate point (4,12) was 60 cd/m², and non-uniformity in luminance in terms ofmaximum/minimum ratio is 1.98 times, which indicates that thenon-uniformity is suppressed to a level lower than 2 times.

[0055] To obtain more uniform luminance distribution, the method is alsoeffective in which the luminance is once corrected by the patterndesigning method described above, further the luminance distribution ismeasured in the same way, distribution of standardized values isobtained, and the luminance distribution obtained first is subjected tosecondary correction. By repeating correction a required number of times(n), homogenous and high luminance can be obtained on the light-emittingsurface of the light guiding member.

[0056] An example of a single color light source in which one LED chipis packaged in one dot-like light source has been described above, andin the case where a plurality of LED chips are provided in one LEDpackage, regardless of which of the plurality of LED chips emits light,as a geometrical and positional relation between the light guidingmember and the LED chip is kept substantially constant, so that thelight emitted from the LED chip is homogeneously distributed from thelight-emitting surface of the light guiding member. Namely alight-emitting surface with the generally homogeneous luminancedistribution is obtained. For instance, LED chips for emitting lightswith red (R), green (G), and blue (B) colors are packaged in the sameLED package so that a distance between the two adjoining chips is about1 mm, and the package is used as a light source. This display lamp canbe turned ON switching between the R, G, and B colors, and further LEDSfor two or three colors can simultaneously be turned ON to emit lightwith a mixed color. As the distance between dot-like light sources issmall as described above, change of non-uniformity in the luminance ateach light-emitting point for the same dot pattern can visually berecognized little. The plurality of chips may emit light having the samecolor. In this case, displays can be provided by changing the luminancelevel.

[0057] In the embodiment described above, a longer edge of therectangular flat light guiding member is 50 mm, but the length isallowable in the range from about 20 to about 200 mm. The luminancelargely depends on intensity of light emitted from an LED, and when thelight has the intensity similar to that described in the exampledescribed above, the luminance of the displays becomes lower as the sizebecomes larger. Further it is necessary to redesign the dot pattern eachtime the size of the light guiding member is changed.

[0058] Further in the embodiment described above, the light scattererwas formed with circular dots, but a form of the dot is not limited to acircular one. Rather a square pattern which has the same form as thesegment is better because higher consistency between the luminance andthe area ratio can be realized (in this case, the lowest luminancesection is the entire segment printed surface, and a percentage ofluminance against the minimum luminance is allocated to each segment).Further a form of a light scatterer pattern may be changed to many otherones including a rhombic one.

[0059] Further, the screen printing method was used as a method offorming a light scatterer in the embodiment described above, but it ispossible to form a light scatterer pattern by applying various types oforganic or inorganic light scattering materials (mainly white paint)onto a bottom surface of a light guiding member by such a method as theoffset printing or ink-jet printing.

[0060] In stead of applying a highly reflective material as describedabove, the light may be reflected by roughing a bottom surface of thelight guiding member. The methods which can be used make rougher abottom surface of a light guiding member include mechanically roughingthe bottom surface, forming a number of fine irregularities, forinstance, by the sandblast method, directly performing processing forgeneration of irregularities to a metal mold for injection molding andtransferring the irregularities to a bottom surface of the light guidingmember during molding, and making linear grooves by means of SCmachining. Basically these methods are based on utilization of the lightscattering and reflection effect provided by roughing the bottomsurface.

[0061] Second Embodiment

[0062]FIG. 9 shows a plane display lamp according to a second embodimentof the present invention. This display lamp 22 has the configuration inwhich, in the structure shown in FIG. 1, a letter or a sign 34 isprinted on the diffusing sheet 20, or a light shielding plate 36 madefrom a metal or plastics is adhered on the diffusing sheet 20. Also theconfiguration is allowable in which an opaque letter or a sign isprinted or adhered on a glass or transparent glass or plastic sheet.With the configurations as described above, it is possible to obtain aplane display lamp capable of displaying a letter or a sign.

[0063] Third Embodiment

[0064] In the first and second embodiments of the present invention, twodot-like light sources, namely two LEDs are provided at a centralposition of each of the side faces in the side of the shorter edges ofthe light guiding member opposing to each other respectively. However,realizing a plane display lamp insuring high uniformity in luminanceonly with one dot-like light source provides a number of advantagesincluding cost reduction, easiness of the manufacture, reduced powerconsumption, and the like.

[0065] In the first embodiment, when the LED 14 b is removed, theluminance distribution on the light-emitting surface corresponding tothat shown in FIG. 3 is as shown in FIG. 10. In FIG. 10, the regionsseparated from each other with contour lines 38 are indicated by thesigns 38-1, 38-2, 38-3, 38-4, 38-5, and 38-6. As only one piece of LEDis provided, light from the LED does not fully reach corners of thelight-emitting surface in the side of the LED 14, so that the luminancein the region 38-6 is insufficient. This phenomenon occurs becauseintensity of the light emitted from the LED is higher at positionscloser to the center of the light-emitting section as well as to thenormal line thereto and lower at positions farther from the center (inthe direction at 90 degrees against the normal line and does notdirectly reach corners of the light guiding member in the LED side. Theluminance in the highest luminance region was 7 times or more higher ascompared to that in the lowest luminance region where the light from theLED does not fully reach.

[0066] In the plane display lamp with only one LED provided therein, adot pattern is printed on a bottom surface of the light guiding memberby the circular dot pattern formation method similar to that employed inthe first embodiment. In this case, even when white paint is applied tothe entire bottom surface of the light guiding member in theinsufficient luminance region 38-6 and at the same time 0.09 mm² dot isprovided per 1 mm² (area ratio is 11) in the highest luminance region18-1, the luminance in the highest luminance region 28-1 is 2 times ormore higher as compared to that in the insufficient luminance region28-6, so that it is difficult to obtain a light-emitting surfaceinsuring uniform light emission.

[0067] To solve the problem described above, the present inventorsinvestigates the possibility of development of a means capable ofreducing non-uniformity in the luminance on a light-emitting surface ofa light guiding member even in a plane display lamp with only one LEDprovided therein. At first, the state of light in a light guiding memberis discussed in detail below. The light coming into a light scattererpattern printed on a bottom surface of the light guiding member isscattered by this light scatterer pattern in the light guiding member.On the other hand, the light introduced to a region where a lightscatterer pattern is not present follows the Snell's law, and is totallyreflected on a surface of the light guiding member, or is emitted to aspace outside the light guiding member. Namely assuming that an anglebetween a normal to a surface of a light guiding member and a light beaminside the light guiding member is ψ and a refraction index of the lightguiding member is n, the light beam follows the Snell's law, and istotally reflected on a surface of the light guiding member when n·sin ψis larger than 1. So long as this condition is satisfied, the light beamis repeatedly reflected inside the light guiding member, and does not goout of the light guiding member. When n·sin ψ is smaller than 1, thelight beam is emitted from the light guiding member. In the case of aplane display lamp, as each face excluding the light-emitting surface iscovered with a white casing, the light emitted from the light guidingmember is partially reflected by the white casing and goes back into thelight guiding member.

[0068] As described above, behaviors of light in a light guiding memberincluding a casing are very complicated, but a percentage of the lightscattered by the light scatterer and emitted therefrom is substantiallyhigh. Therefore, it has been found that, by appropriately selecting asection where the light scatterer is to be formed, it is possible toprovide uniform luminance distribution on a light-emitting surface.

[0069] Based on this recognition, the inventors have provided a lightscatterer on each side face of a rectangular light guiding member. Itcan be expected that, by providing a light scatterer on a side face ofthe light guiding member close to the insufficient luminance region38-6, the light otherwise emitted from the side face to the outside caneffectively be utilized. When actually a printed pattern is provided onall over the necessary side face, the luminance in the insufficientluminance region remarkably increased.

[0070] Printing light scatterer patterns on both a bottom surface andside faces of a light guiding member required a very complicatedmanufacturing process. So the present inventors have devised the methodin which a printed pattern is formed only on a bottom surface of a lightguiding member and is not required to be formed on side faces thereon.

[0071] This method is described below with reference to the thirdembodiment. FIG. 11 and FIG. 12 shows a light guiding member 40according to this embodiment of the present invention. FIG. 11 is aperspective view showing the light guiding member 40 according to thisembodiment. FIG. 12 is a view showing a top surface and side faces ofthe light guiding member 40 according to this embodiment.

[0072] The light guiding member 40 has a plate-like form as shown inFIG. 11, and comprises a bottom surface 44 opposite to the top surface,and side faces 50, 52, 54, and 56. The three side faces 50, 52, 54excluding the side face 56 are inclined. Namely the side faces areinclined so that the distance between two opposing side faces becomeslarger as it goes farther from the top surface 42 and closer to thebottom surface 44. The inclination of each of the three side face inthis embodiment is, for instance, 45 degrees.

[0073] Provided on the inclined side face 50 is a recessed section 58extending from a top surface of the light guiding member 40 to a bottomsurface thereof at a center of the inclined side face. The recessedsection 58 should preferably have a form allowing installation of theLED 14 so that the light emitted from the LED 14 and advancing straightover the shortest distance will be introduced onto the opposite sideface 56 at substantially right angles.

[0074] As described above, when there is provided only one LED, there isthe region 38-6 where the luminance 38-6 is insufficient (Refer to FIG.12). To overcome the problem, highly reflective ink is printed over theentire bottom surface of the light guiding member 40 with one LEDprovided therein as shown in FIG. 11 is prepared, and a plane displaylamp for designing a light scattering pattern is assembled using thislight guiding member to design a circular dot pattern. The circular dotpattern designed as described above is printed on a bottom surface ofthe light guiding member just below a top surface thereof. In addition,a pattern 62 is printed all over in part on bottom surfaces just belowthe inclined side faces 50, 52, 54 and close to the insufficientluminance region 38-6 respectively. With the printed patterns 62, theluminance in the insufficient luminance region 38-6 is increases to alevel equal to or higher than that in the region 38-5, and the planedisplay lamp insures uniformity in the luminance with the light from oneLED 12.

[0075] As a variant of the light guiding member as described above, thefollowing one is conceivable. In the light guiding member 40 shown inFIG. 11, the same effect can be achieved, for instance, by printing apattern on the entire not-inclined side face 56. Further it is alsopossible to transfer a light-scattering pattern by means of a metal moldonto side faces of the light guiding member in place of printing thepattern on each side face thereof.

[0076] Further it is also possible to deform a light guiding member, forinstance, by inclining only a side face on which an LED is mounted, orby also inclining a side face with no LED mounted thereon and oppositeto the side face with an LED mounted thereon. In addition, a side faceof the light guiding member may be inclined not over the full lengthfrom the top surface to the bottom surface, but for instance from thetop surface to the middle of each side face.

[0077] Fourth Embodiment

[0078] Next a fourth embodiment of the present invention is describedwith reference to FIG. 13. This embodiment is a variant of theembodiment described with reference to FIG. 11 and FIG. 12, and in thisembodiment, the LED 14 is provided on a bottom surface of the lightguiding member just below the inclined side face of the light guidingmember 40. The light emitted from the LED 14 advances goes into thelight guiding member from its bottom surface and is reflected andscattered by the inclined side face 50 or a casing (not shown) and isled to inside of the light guiding member 40.

[0079] Further to raise the efficiency in reflection and scattering, arecessed section 70 having a curved surface for reflecting or scatteringlight is provided on the inclined surface 50 just above the LED. Theluminance distribution on the light-emitting surface of the planedisplay lamp could be uniformed by making a form of the surface of thisrecessed section 70 aspheric.

[0080] Fifth Embodiment

[0081] A fifth embodiment of the present invention is described withreference to FIG. 15. This embodiment is a variant of the embodiment ofthe present invention shown in FIG. 11, and in this embodiment, a holefor engagement is provided inside the light guiding member 40 adjacentto the inclined side face 50, and a dome-shaped resin-shielded type ofLED 72 is provided in the hole 40. The light emitted from theresin-shielded type of LED 72 is reflected and scattered by the inclinedside face 50 or a casing (not shown) and is led to inside of the lightguiding member 40.

[0082] Inclining one or a plurality of side faces of a light guidingmember like in the third to fifth embodiments provides advantages alsoin the manufacture of a light guiding member. A light guiding member isgenerally manufactured by molding a transparent resin such as acrylicresin into a desired form with a metal mold. When one or a plurality ofside faces of a light guiding member are inclined, also a side face ofthe metal mold is inclined, and after molding is completed, byseparating the molded light guiding member from the bottom side of themetal mold, side faces of the metal mold hardly contact side faces ofthe light guiding member, so that the side faces of the light guidingmember are little damaged.

[0083] In the third to fifth embodiments of the present invention, onlythe light guiding member is shown, and a casing covering the lightguiding member is not shown. This casing functions also to reflect lightemitted from the light guiding member, so that the color is typicallywhite like in the first embodiment, but the color may be other one like,for instance, silver. In the case of a single color light source, anappropriate color can be selected according to the wavelength of thelight emitted from the light source.

INDUSTRIAL APPLICABILITY

[0084] With the present invention, as an LED, which is a semiconductorchip, is used as a light source, a high voltage circular as a drivecircular is not necessary, and also a plane display lamp with longlifetime and high reliability can be realized. Further as one or twocompact size LEDs may be used therein, a small size, light weight, andcompact plane display lamp with low power consumption can be obtained.When two LEDs are used in the plane display lamp as described above, alight guiding member with a light scattering dot pattern formed thereonby the pattern formation method according to the present invention isused, so that the uniform luminance distribution can be obtained.Further when a light guiding member with inclined side face(s) is used,alight scattering pattern is provided on the side faces adjacent to theinsufficient luminance region or on a bottom surface of the lightguiding member just below the side faces, so that the uniform luminancedistribution can be obtained.

1. A plane display lamp comprising: a plate-like light guiding member; abottom plate covering a bottom surface of said light guiding member; acasing frame covering side faces of said light guiding member; adiffusing sheet covering a top surface of said light guiding member; anddot-like light sources provided on one side face or on two side facesopposite to each other of said light guiding member and emitting lightto said light guiding member; wherein a bottom surface of said lightguiding member has a light scattering means.
 2. A plane display lampcomprising: a plate-like light guiding member having a rectangular topsurface, a rectangular bottom surface, and four side faces each verticalto said top surface as well as to said bottom surface; a bottom platecovering a bottom surface of said light guiding member; a casing framecovering side faces of said light guiding member; a diffusing sheetcovering a top surface of said light guiding member; and dot-like lightsources provided on one side face or on two side faces opposite to eachother of said light guiding member and emitting light to said lightguiding member; wherein a bottom surface of said light guiding memberhas a light scattering means.
 3. A plane display lamp comprising: aplate-like light guiding member having a rectangular top surface, arectangular bottom surface, and four side faces, at least one of whichhas an inclination extending from said top surface toward said bottomsurface; a bottom plate covering a bottom surface of said light guidingmember; a casing frame covering four side faces of said light guidingmember; a diffusing sheet covering a top surface of said light guidingmember; and one dot-like light source emitting light to said lightguiding member; wherein a bottom surface of said light guiding memberhas a light scattering means.
 4. The plane display lamp according toclaim 3, wherein said one dot-like light source is installed in arecessed section provided on said one inclined side face.
 5. The planedisplay lamp according to claim 3, wherein said one dot-like lightsource is provided on a bottom surface of said light guiding member justbelow said one inclined side face.
 6. The plane display lamp accordingto claim 5, wherein a recessed section having a curved surface forreflecting and scattering light is provided on said inclined side faceabove said one dot-like light source.
 7. The plane display lampaccording to claim 3, wherein said one dot-like light source is providedinside said light guiding member just below said one inclined side face.8. A plane display lamp comprising: a plate-like light guiding memberhaving a rectangular top surface, a rectangular bottom surface, and fourside faces, three of which have an inclination extending from said topsurface to said bottom surface in a portion thereof; a bottom platecovering a bottom surface of said light guiding member; a casing framecovering four side faces of said light guiding member; a diffusing sheetcovering a top surface of said light guiding member; and one dot-likelight source emitting light to said light guiding member; wherein abottom surface of said light guiding member has a light scatteringmeans.
 9. The plane display lamp according to claim 8, wherein said onelight source is installed in a recessed section provided on a centralone of said three inclined side faces.
 10. The plane display lampaccording to claim 8, wherein said one dot-like light source is providedon a bottom surface of said light guiding member just below said centralinclined side face.
 11. The plane display lamp according to claim 10,wherein a recessed section for reflecting and scattering light isprovided on said inclined side face just above said one dot-like lightsource.
 12. The plane display lamp according to claim 8, said onedot-like light source is provided inside said light guiding member justbelow said central inclined side face.
 13. The plane display lampaccording to any of claims 3 to 7, wherein a light scattering means isprovided on side faces adjacent to a region with insufficient luminancewhere the light emitted from said dot-like light source does not fullyreach.
 14. The plane display lamp according to any of claims 8 to 12,where a light scattering means is provided on a bottom surface of thelight guiding member just below each of the side faces adjacent to aregion with insufficient luminance where the light emitted from saiddot-like light source does not fully reach.
 15. The plane display lampaccording to claim 1, 2 or 3, wherein said dot-like light source is alight-emitting diode.
 16. The plane display lamp according to claim 1, 2or 3, wherein said bottom plate and casing frame are white.
 17. A methodof forming a light scatterer dot pattern on a bottom surface of aplate-like light guiding member in a plane display lamp including saidlight guiding member; a bottom plate covering a bottom surface of saidlight guiding member; a casing frame covering side faces of said lightguiding member; a diffusing sheet covering a top surface of said lightguiding member; and dot-like light sources provided on one side face oron two side faces opposite to each other of said light guiding memberand emitting light to said light guiding member; said method comprisingthe steps of: a) forming a light scatterer homogeneously all over abottom surface of a light guiding member for assessment having the samesize and made from the same materials as said light guiding member; b)assembling said plane display lamp; c) measuring luminance at eachcoordinate point on a light-emitting surface of said plane display lamp;d) converting the measured luminance at each coordinate point throughinverse-proportional operation to standardized value using the lowestvalue in a distribution the luminance as a reference; e) calculating anarea of a light scatterer dot at each coordinate point from thestandardized value obtained from the conversion in the previous step;and f) forming a pattern of light scatterer dots each area thereof iscalculated in the previous step on a bottom surface of a light guidingmember different from that for assessment.
 18. The method according toclaim 17 further comprising the steps of: g) assembling said planedisplay lamp using the light guiding member prepared in the step f; h)measuring luminance at each coordinate point on the light-emittingsurface of said plane display lamp; i) converting the measured luminanceat each coordinate point through inverse-proportional operation tostandardized value using the lowest value in a distribution of theluminance as a reference; j) reflecting a difference from thestandardized value obtained in the step d when calculating an area of alight scatterer dot at each coordinate point from the standardized valueobtained through the conversion step; k) forming said calculated lightscatterer dot pattern on a bottom surface of another light guidingmember further different from the different light guiding member in thestep f; and l) repeating the operating sequence from the step g to thestep k n times (n is an integer larger than 0) to the light guidingmember prepared in the step k.
 19. The method according to claim 17,wherein the conversion to the standardized value in the step d isperformed through the following equation: Standardized value=Lowestluminance value/measured luminance at each coordinate point.
 20. Themethod according to claim 18, wherein conversion to the standardizedvalue in the steps d and i is performed through the following equation:Standardized value=Lowest luminance value/measured luminance at eachcoordinate point.
 21. The method according to claim 20, wherein saidlight scatterer dot has a form satisfying the following condition:Standardized value=A×dot area wherein A indicates a proportionalconstant.
 22. The method according to claim 20, wherein, when said lightscatterer dot is a circular dot, the radius thereof is calculated in thesteps e and j through the following equation: Standardizedvalue=A×π×(radius)² wherein A indicates a proportional constant.
 23. Themethod according to any of claims 17 to 22, wherein said light scattererdot pattern is formed so that a reflection factor is minimum at aposition adjacent to said dot-like light source.
 24. The plane displaylamp according to claim 1, 2 or 3, wherein a light scatterer dot patternis formed on a bottom surface of said light guiding member just below atop surface thereof by the method according to claim
 21. 25. The planedisplay lamp according to claim 24, wherein said bottom plate and casingframe are white.
 26. the plane display lamp according to claim 1, 2, or3, wherein a light scatterer dot pattern is formed on a bottom surfaceof said light guiding member just below a top surface thereof by themethod according to claim
 22. 27. The plane display lamp according toclaim 26, wherein said bottom plate and casing frame are white.